In the fields of optical communication and optical information processing, nonlinear optical elements play an important role. Nonlinear optical materials used for the nonlinear optical elements are materials which perform very important functions on optical signal processing, such as photomixing generating the sum of frequencies of two kinds of incident light different from each other in frequency, optical parametric producing two kinds of light different from each other in frequency, the Pockels effect and the Kerr effect changing the refractive index of optical media, and secondary harmonic generation (SHG) or tertiary harmonic generation (THG) (i.e., conversion of incident light to a secondary or tertiary harmonic light). As such nonlinear optical materials, inorganic and organic materials have hitherto been discovered.
As the inorganic nonlinear optical materials, the crystals of inorganic compounds such as KDP (KH.sub.2 PO.sub.4) and lithium niobate (LiNbO.sub.3) have been known. However, they were not enough to satisfy fully the requirements.
On the other hand, the organic nonlinear optical materials have recently been noted as materials for new optical elements in the field of optoelectronics, and increasingly investigated year by year. In particular, many investigations for search of materials have been made of organic compounds having .pi. electron conjugated systems, because of the high performance and high speed responsibility of their molecular substances.
In general, it has been known that the crystals of the organic nonlinear optical materials are about 10 to 100 times larger in the coefficient of SHG, about 1,000 times higher in optical response speed and also greater in threshold to optical damage than the crystals of the inorganic nonlinear optical materials.
The organic nonlinear optical materials previously known include 2-methyl-4-nitroaniline, m-nitroaniline, N-(4-nitrophenyl)-L-prolinol, 2-acetylamino-4-nitro-N,N-dimethylaniline, 4-dimethylamino-4'-nitro-stilbene, 4'-dimethylamino-N-methyl-4-stilbazolium methyl sulfate and 4'- methylbenzylidene-4-nitroaniline. The nonlinearity of these organic compounds having the .pi. electron conjugated systems is caused by the interaction of laser beams as electromagnetic waves with .pi. electrons of the organic compounds, and this interaction can be further increased by introducing electron attractive substituents and electron donative substituents into the .pi. electron conjugated systems.
In such organic compounds, dipole moments are generally increased, and dipole-dipole interactions in crystallizing become strong, so that central symmetric crystals having a structure in which dipoles of two molecules counteract each other are liable to be formed. There is the problem that the secondary nonlinear optical effect which is important in their applications is not expressed in such central symmetric crystals. In order to put central symmetry into disorder which poses a problem on expression of the nonlinearity in the crystal state, it has been proposed that substituents having hydrogen bonding ability or optically active substituents having asymmetric carbon atoms are introduced into the organic compounds of the .pi. electron conjugated systems.
Characteristics generally required for the materials for nonlinear optical elements include the degree of nonlinearity, light permeability, anti-laser damage strength, crystallinity, phase matching property, processability, mechanical strength and hygroscopic property.
It has been very difficult to select a material satisfying the practically required characteristics as described above from the materials for the organic nonlinear optical elements which has hitherto been known.